Your boiler has shut down because the exhaust gases are getting too hot, which is a safety measure to prevent damage to the internal components.

What causes this fault?

This fault occurs when the internal safety sensors detect that the waste gases leaving your boiler are reaching dangerous temperature levels. The most common reason is a buildup of limescale or debris inside the main heat exchanger, which prevents heat from being transferred to your water and instead sends it out through the flue. It can also be triggered by a faulty sensor giving an incorrect reading or a lack of water flow through the system.